[string.classes]
Change: basic_string requirements no longer allow reference-counted
strings
.  Rationale: Invalidation is subtly different with reference-counted strings
.  This change regularizes behavior for this International Standard
.  Effect on original feature:
Valid C++ 2003 code may execute differently in this International Standard
.  Rationale: Allow small-string optimization
.  Effect on original feature:
Valid C++ 2003 code may execute differently in this International Standard
.  Some 
const member functions, such as 
data and 
c_str,
no longer invalidate iterators
.